1. Field of the Invention
The present invention relates to resist removal for semiconductor substrates. In particular, the present invention relates to exposing resist prior to developing the resist, in order to remove the resist from the wafer, especially during rework.
2. Description of the Related Art
Clean wafers are very important in obtaining high yields for semiconductor fabrication. As described in Section 15 of Silicon Processing for the VLSI Era, Volume I, by S. Wolf and R. N. Tauber, there are two major sources of wafer contamination, particulates and films. Particulates are bits of material that are on the wafer surface, and can include silicon dust, quartz dust, atmospheric dust, and particulates from clean room personnel and processing equipment, just to name a few of the possible sources of particulate contamination. Film contamination corresponds to layers of foreign material on a wafer surface, and can include solvent residues, such as acetone, or photoresist developer residues from dissolved photoresist in the developer, just to name a few of the possible sources of film contamination.
The Wolf et al. reference states that "chemical cleaning and photoresist stripping operations used to remove film contamination have also been identified as significant sources of particle contamination." It is suggested in the Wolf et al. reference that "ultra-pure chemicals be utilized together with in-line point-of-use microfiltration for both chemical cleaning and resist stripping procedures."
The Wolf et al. reference discusses various techniques for photoresist removal, including etching (wet and dry), ion implantation, lift-off processes, high temperature postbake, or simple removal of misaligned resist patterns for reimaging after development and inspection ("rework"). The primary objection of resist removal is to insure that almost all of the photoresist is removed quickly as possible without harming any underlayers.
To perform this task, organic strippers have been utilized, such as phenol-based strippers. The problem with such strippers is that they will attack the metal underlayers, causing damage to the semiconductor substrate. Also, oxidizing-type strippers have been used, such as heated solutions of H.sub.2 SO.sub.4 and an oxidant. Ammonium persulfate (APM) is typically used to maintain baths of consistent composition. As another alternative to resist removal, dry etching of resist can be performed, using oxygen plasmas in plasma etching equipment.
While each of the above-mentioned techniques for resist removal is effective to remove most of the resist, these removal techniques leave behind some resist or residue, or they damage the film underneath by undesired chemical reactions. In some cases, the temperature during oxygen plasma treatment is so high that it causes the underlying film to change. This detrimental situation due to the left-behind residue is highlighted upon deposition of a subsequent film, whereby the residues show up as bumps, which can result in yield loss.